During the drilling, completion, servicing, or evaluation of an oil or gas wellbore or the like, situations are encountered wherein a downhole tool or a component of drill string or other conveyance becomes lodged in the wellbore. When the static force necessary to move a component exceeds the rig's capabilities or the tensile strength of the conveyance, the component is stuck and can no longer be moved or rotated. A jar is a tool that may be prepositioned within the string or other conveyance, such as wireline, e-line, slickline, etc., to free any component which may become stuck. Jars may also be used to shear pins, push or pull tools, actuate tools, et cetera.
A jar operates by releasing stored potential energy. Jarring is the process of converting potential energy into kinetic energy concentrated at a given point. In a typical jar, the potential energy available comes from over-pull (tensile) or set-down (compressive) forces applied to the drill pipe at the surface.
A typical jar may include a mandrel, which slides within a sleeve, and a detent mechanism. The mandrel functions as a hammer, and the sleeve functions as an anvil. The detent mechanism restricts the movement of the mandrel before releasing it (“firing”), so that sufficient potential energy is accumulated and transferred to the mandrel to cause, upon firing, the mandrel to rapidly move and strike the sleeve. This impact creates an impulse and the kinetic energy is transmitted as shock wave that travels up and down the tool string, drill string, or other conveyance to free a stuck tool or pipe, to shear pins, or to perform some other desired function.
A jar tool may be a double acting jar that can provide jarring force both upwards and downwards. The separate functions of jarring upward or downward may be accomplished in any sequence; that is, up only, down only, or alternately up and down. A jar may be classified as either of two types based on the detent mechanism: hydraulic and mechanical.
A hydraulic jar moves a piston with a fluid-filled hydraulic cylinder. Fluid passes from one side of the piston to the other through an orifice, triggering valve, or similar restriction which initially limits flow to create a time delay during the loading phase and then opens the flow path to trip the detent mechanism and fire the jar. In some hydraulic jars, the pressure piston must move a predetermined distance in order to bypass the restriction or open the triggering valve. The built-in delay is designed to allow the operator sufficient time to apply the desired tensile or compressive force to the drill string before the flow restriction is cleared or the triggering valve is opened. Therefore, varying the metering rate of the fluid through the restriction varies the magnitude of impact.
In contrast, a mechanical jar is actuated using a series of springs, locks, and rollers with release mechanisms. A mechanical jar fires upward at a preset tensile force and downward at a preset compressional force, which normally exceed the forces reached during drilling. Firing does not depend on the duration of the loading phase. A mechanical jar is typically either non-adjustable and made to deliver a preset amount of jarring force, or field-adjustable allowing setting at the surface before the jar is run into the wellbore.